In a typical laser resonator, an active laser medium is placed in an optical resonator or cavity comprising a set of mirrors. The active laser medium, being a gas, a liquid or a solid, or a combination thereof, contains the active atoms, ions, molecules or particles of matter that will generate the laser beam. This active medium is excited through transfer of external energy to active elements of the medium by some means such as an electrical discharge, an external source of light, a chemical reaction or any source of energy. The elements, which may be active atoms, ions, molecules or particles of matter, radiate some energy in the form of electromagnetic radiation or light. If this same light is partly or totally re-injected into the excited active laser medium, it stimulates the emission of additional light according to the principle of the stimulated emission process. As illustrated in FIG. 1, this is typically done by placing the active laser medium 1 inside an optical resonator or cavity formed by mirror 2 and a partially reflecting mirror 3. The generated laser beam is emitted through the mirror 3.
Many different configurations of optical resonators including wave-guide, ring, and stable and unstable configurations have been designed to achieve better quality laser beams or laser beams with specific characteristics such as single transverse or spatial mode, single longitudinal or frequency mode, or to better control some of the characteristics of the laser beam. Laser sources have also been used in a cascade configuration, where one well controlled laser source is used to control the subsequent ones, such as in a Master-Oscillator-Power-Amplifier or a seed-injected laser.
It remains that in some lasers, some minute inhomogeneities or perturbations inside the laser resonator can greatly deteriorate some of the characteristics of the laser beam. Such an example is a solid-state laser in which the active laser medium is a solid single crystal containing some active ions. In such a system, some minute optical inhomogeneities due for example to minute optical imperfections in the active crystal or other optical elements, or minute inhomogeneities in the excitation process and non-uniform excitation of the active laser medium, can lead to some unpredicted perturbations in the laser beam. This can sometimes generate some relatively strong variations in the spatial or cross-sectional or temporal intensity profile of the laser beam known as "hot-spots." These "hot-spots" can be described as local areas in the cross section of the laser beam or in the temporal intensity profile where the intensity of the laser beam greatly exceeds its average or expected value. Such "hot-spots" are usually undesirable, as they can lead to unwanted and unpredictable optical damages in the laser source itself or in other components, or unwanted and unpredictable effects in the use of the laser beam.